A communication system providing a voice call service and a video call service based on an Internet Protocol (IP) starts the services by transmitting information on call conditions such as the type of image compression technology, a bit rate, the size of an image, and the like. The information on the call conditions is transmitted to a counterpart terminal by a terminal, which initiates a call by mutually negotiating the call conditions between the two terminals.
FIG. 1 is a signal flow diagram illustrating a general process of negotiating a session for a video call service based on an IP according to the related art.
Referring to FIG. 1, a transmission terminal 100 describes call conditions, which the transmission terminal 100, on a Session Description Protocol (SDP) offer in operation 101, takes the SDP in a Session Initiation Protocol (SIP) INVITE message, and transmits the SIP INVITE message to a reception terminal 140 in operation 102. The SIP message including the SDP offer is transferred to the reception terminal 140 via an IP Multimedia Subsystem (IMS) entity 120 and Policy and Charging Rules Function (PCRFs) 110 and 130 in a wired/wireless network. The SIP message guarantees call quality, such as a Voice over Long Term Evolution (VoLTE), and is transferred to the reception terminal via a management server in the Internet.
The reception terminal 140, which has received the SIP message including the SDP offer of the transmission terminal 100, reviews an image processing ability, a service policy, a bit rate, the size of an image, etc. of video COder/DECoders (CODECs) mounted therein. The reception terminal 140 writes an SDP answer including information on call conditions which the reception terminal 140, takes the SDP answer 103 in a SIP 183 session progress, and transmits the SIP 183 session progress to the transmission terminal 100 in operation 104.
When the transmission terminal 100 receives the SDP answer of the reception terminal 140 and accepts the SDP answer, a mutual confirmation process is performed therebetween, and then a call starts. When the transmission terminal 100 does not accept the SDP answer of the reception terminal 140, the transmission terminal 100 configures a new SDP offer. The transmission terminal 100 takes the new SDP offer in a SIP UPDATE message, and transmits the SIP UPDATE message to the transmission terminal in operation 105, and the reception terminal 140 rewrites an SDP answer, takes the rewritten SDP answer in a SIP 200 OK message, and transmits the SIP 200 OK message to the transmission terminal 100 in operation 106. When the transmission terminal 100 accepts the second SDP answer of the reception terminal 140, the transmission terminal 100 transmits a SIP Provisional Response ACKnowledgement (PRACK) to the reception terminal 140 as a response in operation 107, the reception terminal 140 transmits a SIP 200 OK message to the transmission terminal 100 as a response thereto, and thereby the negotiations are completed and calling starts in operations 108 and 109. When mutually-acceptable common call conditions have not been negotiated, the transmission terminal 100 or the reception terminal 140 gives up the video call and makes a proposal for a voice call.
FIG. 2 illustrates an example of a detailed configuration of an SDP offer illustrated in FIG. 1 according to the related art.
Referring to FIG. 2, the transmission terminal supports two types of video CODECs of H.264 and H.263. H.264 includes information indicating that an image may be encoded and decoded into four kinds of sizes of 176*144, 240*320, 320*480 and 480*640 at a bit rate of up to 384 kbps and especially an image having a size of 480*640 is desired to be received. Such an image processing ability should exist within a range designated by a value of profile-level-id in H.264. H.263 includes only a proposal to use a bit rate of 57 kbps, in which case, since a profile and a level value, which indicate an image processing ability of H.263, designate only one size of image of 176*144, the size of image need not be negotiated.
In the 4G network represented by the Long Term Evolution (LTE), a bit rate of an image service increases considering the transmission capability of the 4G network. Accordingly, an HD level of H.264 L3.1 is recommended. Further, in the Rich Communication Suite (RCS) adopted by the Global System for Mobile communication Association (GSMA), an image service such as video-sharing in addition to the video call has appeared, and a procedure of negotiating the size of video with respect to a pixel level is introduced, so that it is necessary to negotiate, with a counterpart terminal, a process of compressing an image with respect to a video CODEC, a bit rate, the size of a receivable and compressible image of a terminal, and the like.
FIG. 3 illustrates a detailed configuration of an SDP answer corresponding to FIG. 2 according to the related art.
Referring to FIG. 3, the SDP offer, which is received from the transmission terminal, represents that H.264 video CODEC is selected, a bit rate of 384 kbps is accepted, it is accepted that an image is compressed into the size of 480*640 and then transmitted, and the transmission terminal itself prefers to receive an image having the size of 320*480. This fact implies that the reception terminal can receive and restore an image of 320*480 simultaneously while compressing an image of 480*640 and transmitting the image of 480*640 to the transmission terminal.
In FIG. 3, “a=sendrecv” implies that such a compression condition is applied to both the transmission direction and the reception direction in a current session, and when different bit rates are used in the two directions, the reception terminal separately writes compression conditions to which “a=send” and “a=recv” are applied, and performs negotiations. In this way, as the SDP offer is complicated, a period of time consumed for session negotiations increases and it is difficult to predict a result of the negotiation.
There are various resolutions and various material properties of display panels, which are reasons that negotiations between the transmission terminal and the reception terminal make difficult. Display panels of mobile phones, which have mainly employed Liquid Crystal Displays (LCDs) using a Red/Green/Blue (RGB) scheme, are recently being changed to Organic Light Emitting Diode (OLED) displays or panels having various physical characteristics and various pixel structures. From among these, recently, there is trend of using panels having a PenTile™ structure in order to achieve high resolution in the OLED of which a manufacturing process is complex and a yield rate is low. Further, a technology of applying, to the OLED panel, White RGB (WRGB), which has been introduced to enhance white color processing in the LCD, as well as the RGB has been introduced to Television (TV) displays in addition to mobile phones.
An image compressed by a video encoder of a transmission device should be restored to an image suitable for a display of a reception device. When information on characteristics of the display panel of the reception device is not transmitted to the transmission device during a process of negotiating call conditions, image quality may be damaged during a process of restoring an image by the reception device. In a basic process of compressing an image, an image sensor of a camera uniformly samples RGB components and converts the sampled RGB signal into an YCbCr signal representing luminance information and chrominance information. However, since human eyes are more sensitive to luminance than chrominance, the image is compressed while a Cb component and a Cr component are reduced more than a Y component. For example, the YCbCr signal is compressed by a video encoder by sampling from a ratio of 4:4:4 to a ratio of 4:2:2, 4:2:0, 4:1:1, or the like. Further, image quality of a display panel of the reception device is influenced by an input signal such as the RGB and the YCbCr, and each panel has an YCbCR format which is most appropriate for color development ability or a pixel structure. However, since the camera or the video encoder of the transmission device does not know information on the YCbCr format appropriate for the display of such a reception device, the image is compressed irrelevant thereto and is transmitted. Thus, there is a problem in that electric power is consumed and the image quality is damaged while the reception device receives and restores the image, which has been compressed and transmitted by the transmission device, and converts the image to be appropriate for a panel input thereof.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present disclosure.